The present invention is directed to exploration and development of sources of hydrocarbons and particularly to such exploration by acoustic investigations of the properties of the formations traversed by a borehole. More specifically, the present invention is directed to an apparatus and method for measuring formation properties by transmitting an acoustic signal into the formation and for detecting the acoustic signal at a spaced detector after it has propagated through the formation, wherein the apparatus and method include means for reducing tool mode vibration in a sonic logging tool.
In a conventional sonic logging a tool containing an acoustic transmitter and an array of acoustic receivers is lowered into a borehole to measure the propagation of acoustic and elastic waves in earth formation outside the borehole.
Difficulties arise because waves propagating directly along the tool body (i.e., not propagating in earth formation) are also detected by the receiver array. These xe2x80x9ctool wavesxe2x80x9d introduce errors unless their effects can be filtered out. Traditionally, this is dealt with by using a xe2x80x9cslow toolxe2x80x9d, i.e., a tool having a structure designed to slow down the tool waves to permit filtering by time of arrival at the receiver array. Slow tools use slotted sleeves, grooved collars, and attenuation components.
Schlumberger Technology Corporation, the assignee of this application, provides tools having slotted sleeves, grooved collars, and attenuation components to permit filtering by time of arrival at the receiver array. Schlumberger Technology Corporation provides an acoustic wireline tool, the Dipole Shear Sonic Imager Tool (DSI), that operates in a manner substantially as set forth in co-owned U.S. Pat. No. 5,036,945 to Hoyle et al. The above-mentioned U.S. patent application Ser. No. 09/537,836 (US national entry of published international application WO 01/73478 A3 xe2x80x9cDipole Logging Toolxe2x80x9d) is another example of a slow wireline sonic tool. Schlumberger Technology Corporation also provides an acoustic logging while drilling (LWD) tool (the xe2x80x9cISONIC Toolxe2x80x9d) that operates in a manner substantially as set forth in co-owned U.S. Pat. No. 5,796,677 to Kostek et al. Both xe2x80x9cDSIxe2x80x9d and xe2x80x9cISONICxe2x80x9d are trademarks of Schlumberger.
References in active noise control literature include xe2x80x9cActive Control of Sound Radiation from Cylinders with Piezoceramic Actuators and Structural Acoustic Sensingxe2x80x9d, J. Mailliard, C. Fuller, ASA 133rd meetingxe2x80x94Penn State, June 1997; xe2x80x9cCharacteristics of Enhanced Active Constrained Layer Damping Treatments with Edge Elements, Parts 1 and 2. Journal of Vibrations and Acoustics, pp 886-900, Volt 120, October 1998; xe2x80x9cActive Control of Sound and Vibrationxe2x80x9d by C. R. Fuller and A. H. von Flotow, IEEE Control Systems, December 1995, pp. 9-19; xe2x80x9cDesign of Active Noise Control Systems With the TMS320 Familyxe2x80x9d, S. M. Kuo et al, Texas Instruments Technical Report SPRA042, June 1996.
The invention provides a method and an apparatus for reducing tool borne noise in a sonic logging tool. The preferred method includes distributed active cancellation of tool mode vibration using digital filters, preferably finite impulse response filters (FIR filters). The preferred apparatus includes an acoustic logging tool that can be positioned within a fluid-filled borehole, the tool having an axially distributed active vibration control system with actuator assemblies coupled to cancel tool mode vibrations at each acoustic receiver along the receiver section of the tool.
The method reduces tool borne noise in a sonic logging tool having an acoustic transmitter and an axial array of acoustic receivers. The method comprises canceling tool mode vibration using an axially distributed active control system The location of each acoustic receiver defines a station. Actively canceling tool mode vibration includes applying a force to the tool at each station.
In a preferred embodiment, force is applied in feedback mode based on measuring tool mode vibration at each station. The value of the force at each station is established by a real-time computational algorithm using a feedback model. The feedback includes constant azimuthal weighting to accommodate monopole tool mode vibration, or sinusoidal azimuthal weighting to accommodate dipole tool mode vibration or other appropriate weighting to accommodate other tool mode vibration. The computational algorithm includes digital signal processing of signals from a vibration sensor assembly. Preferably, digital signal processing of vibration sensor signals is through digital FIR filters. The force applied at a given station is a function of tool mode vibration measured at that station and the predetermined values of a set of FIR filter coefficients. Each FIR filter has at least two filter coefficients. Preferably each FIR filter is a 7-order FIR filter having seven coefficients. The predetermined values of the seven coefficients of each FIR filter are determined by an iterative optimization technique. The preferred iterative optimization technique includes: a) initializing the value of the seven coefficients of the FIR filter in each elemental feedback loop; b) firing the acoustic transmitter and recording vibration at every element of every station of a tool for a period of time with the tool in a water tank and the feedback system running; c) summing the squares of the recorded data; d) modifying filter coefficients using an optimization technique to minimize sum of squares; e) repeating steps b)-d) until the sum is sufficiently small; and f) saving optimized values of filter coefficients.
In an alternative embodiment, each force is applied in feed-forward mode. The value of each force is established by a real-time computational algorithm in an active feed-forward control system. The feed-forward model includes constant azimuthal weighting to accommodate monopole tool mode vibration, or sinusoidal azimuthal weighting to accommodate dipole tool mode vibration. The force applied at a given station is a function of transmitter output and the predetermined values of the FIR filter coefficients. Each FIR filter uses a number of coefficients, up to a number as large as the number of time samples used to define the acoustic transmitter firing pulse. Preferably each FIR filter is a 100-order FIR filter having a hundred coefficients. The predetermined values of the 100-order FIR filter coefficients are determined by using an algebraic technique to solve for zero vibration at all stations. The algebraic technique includes: a) firing the acoustic transmitter with a unit pulse and recording tool vibration at every element of every station with the active control system not running; b) applying a unit pulse at active elements of a first station having active elements, applying the unit pulse with the appropriate azimuthal weighting, including constant azimuthal weighting for monopole configuration, including sinusoidal azimuthal weighting for dipole configuration, and recording tool vibration at every element of every station, with the active control system not running; c) repeating b) for every station having active elements; d) using all the tool vibration recordings from a)-c) to compute a value for every FIR filter coefficient so that the resulting tool vibration at all stations is zero when the feed-forward control system is running; and e) saving computed values of filter coefficients.
The invention provides an acoustic logging tool that can be positioned within a fluid-filled borehole. The tool includes an elongated cylinder defining an axis, an acoustic transmitter mounted to the cylinder, an axial array of acoustic receivers mounted along the cylinder; and an axially distributed active vibration control system. The control system includes at least one cylindrical reaction mass co-axially mounted within the cylinder, and an axial array of actuators coupled to exert force between said at least one cylindrical reaction mass and the cylinder.
The location of each acoustic receiver defines a station. The axial array of actuators includes one actuator assembly located at each station. Each actuator assembly includes multiple actuators in an azimuthal array. Each actuator includes multiple actuator elements, in a preferred embodiment multiple stacked actuator elements.
The control system includes a plurality of electrical outputs. Each actuator element is wired to receive a corresponding one of the plurality of electrical outputs.
The acoustic logging tool further includes an axial array of vibration sensor assemblies, one vibration sensor assembly located at each station. Signals from sensor elements in a sensor assembly are summed using appropriate weighting for monopole or dipole to generate a tool vibration output which is coupled to the active control system.